The invention relates to determining the position of mobile terminals by satellites.
In the present context, the expression “mobile terminal” refers equally to devices (or receivers) dedicated exclusively to satellite positioning and communication terminals equipped with a satellite positioning device, for example mobile telephones or personal digital assistants (PDA), where applicable of the communicating type.
As the person skilled in the art knows, satellite positioning comprises two steps. In the first or acquisition step, the mobile terminal in question determines the pseudo-random codes modulating signals coming from satellites that are “in view” and belong to a constellation of positioning satellites relative to a reference time generally called the “system time”. The signals received from the satellites in view are “compared” to signal replicas resulting from hypotheses as to the system time and as to the timing frequency of the satellites, in order to deduce therefrom the pseudo-random codes that are modulating said received signals, in other words to synchronize the timing clock and frequency of the terminal to the clock and frequency of each satellite in view. To this end, correlation measurements are usually carried out based on pairs of time and frequency hypotheses.
In the present context, the expression “constellation of positioning satellites” refers to a radio navigation satellite service (RNSS) type positioning network, for example the GPS network, the GLONASS network or the future GALILEO network.
In the second step the position of the terminal is determined from the acquired codes and navigation data contained in particular in the signals received. More precisely, this second step may be divided into three sub-steps: a sub-step of determining the propagation times of signals between each of the satellites in view and the terminal from the acquired pseudo-random codes, a sub-step of determining pseudo-distances between the terminal and each of the satellites in view from the navigation data contained in the signals and the propagation times, and a sub-step of determining the position of the terminal from the pseudo-distances (at least by quadrilateration and more generally by finding a numerical solution to an equation with four unknowns and at least four measurements using a method of the least squares type). Four measurements are needed to solve the equation with four unknowns. In some conditions only three measurements are used by fixing one unknown, typically the altitude (Z) of the receiver, or a method may be used based on hybridization with external measurements.
The accuracy of each propagation time, and therefore of each pseudo-distance, directly determines the accuracy of the position. Now, the accuracy of each propagation time depends on the quality with which the pseudo-random codes are acquired from the corresponding received signal, which depends on the quality of said received signal. Consequently, the position determined is generally affected by errors if at least one of the signals received from a satellite in view is of poor quality, which occurs relatively frequently, especially in uneven and congested environments. It may even be temporarily impossible to determine the position of the terminal, even though the signals coming from the other satellites in view are of good quality.